1. Field of the Invention
The present invention relates to an electrophotographic photoconductor and, more particularly to a single-layered organic electrophotographic photoconductor for use in electrophotographic copying machines and printers.
2. Discussion of Background
Inorganic photoconductive materials such as selenium, zinc oxide and cadmium sulfide; organic photoconductive materials such as azo pigments; and amorphous silicon, are conventionally known as photoconductive materials for use in electrophotographic photoconductors. However, the inorganic photoconductive materials have the drawbacks in photosensitivity, thermal stability, durability, impact resistance, and toxicity. For instance, selenium easily tends to crystallize, for example, when inappropriately heated, so that the characteristics readily deteriorate. A photoconductor employing cadmium sulfide or zinc oxide as a photoconductive material has humidity resistance and durability. The organic photoconductive materials can easily be made into a film with much more design freedom than the inorganic photoconductive materials because of the superior film-forming properties of the organic photoconductive materials.
In addition, the organic photoconductive materials are cheaper and cause no environmental pollution problems, so that electrophotographic photoconductors using the organic photoconductive materials have been actively developed in recent years.
Photoconductors comprising such organic photoconductive materials may be classified into two types, that is, a function-separated type photoconductor comprising a charge generation layer and a charge transport layer which are laminated, and a single-layered photoconductor comprising a photoconductive layer in which a charge generating material is dispersed in a binder resin.
In particular, many function-separated type photoconductors are put to practical use because of their superiority in photosensitivity.
For example, a function-separated laminated photoconductor comprising Chloro Diane Blue and a hydrazone compound in combination is proposed in Japanese Patent Publication 55-42380. Charge generating materials for use in such a function-separated laminated photoconductor are disclosed in Japanese Laid-Open Patent Applications 53-133445, 54-21728 and 54-22834; and charge transporting materials are disclosed in Japanese Laid-Open Patent Applications 58-198043 and 58-199352.
When preparing a laminated photoconductor, however, it is necessary to severely control the thickness of a charge generation layer within a range of 0.1 to 1.0 .mu.m for obtaining high photosensitivity. Such thickness control of the charge generation layer is susceptible to the surface conditions of an electroconductive support on which the charge generation layer is provided, and to the ambient conditions while a charge generation coating liquid is applied onto the support. Furthermore, for the preparation of such a laminated photoconductor comprising the charge generation layer and the charge transport layer, at least two coating steps are required. These factors largely affect the yield and manufacturing cost of the photoconductor.
For obtaining a laminated photoconductor with high photosensitivity, a charge transporting material with high electric charge mobility is demanded for use in the charge transport layer. Charge transporting materials with high electric charge mobility are almost limited to compounds which exhibit positive hole transporting characteristics. Therefore, photoconductors for use in practice are necessarily limited to photoconductors of a negative charging type, which are referred to as negatively-chargeable photoconductors.
Most of negatively-chargeable photoconductors are charged by use of corona charge with a negative polarity, so that a large amount of ozone is generated. The thus generated ozone is harmful to the human body, and impairs the photoconductor itself by the reaction with ozone, thereby shortening the life thereof. To prevent the generation of ozone, there are proposed a charging system capable of hindering the generation of ozone, a special system for decomposing the ozone discharged from a charging unit, and a special ozone exhaust system for removing ozone built up in an electrophotographic copying machine. Although some of these systems have been put to practical use, such systems have the shortcomings that they will make electrophotographic process complicated.
On the other hand, the single-layered electrophotographic photoconductors are generally known as positively-chargeable photoconductors. For example, there are proposed a photoconductor comprising polyvinyl carbazole and 2,4,7-trinitro-9-fluorenone in Japanese Patent Publication 50-10496; a photoconductor comprising a polyvinyl carbazole which is sensitized with a pyrylium salt-based pigment in Japanese Patent Publication 48-25658; a photoconductor comprising as a main component a eutectic complex; a photoconductor comprising a charge generating material and a charge transporting material in Japanese Laid-Open Patent Application 47-30330; photoconductors comprising a perylene pigment and a charge transporting material in Japanese Laid-Open Patent Applications 63-271461, 1-118143 and 3-65961; and a photoconductor comprising a phthalocyanine compound and a binder resin in Japanese Laid-Open Patent Application 3-65961.
However, the above-mentioned conventional single-layered photoconductors are still inferior in photosensitivity to the laminated photoconductors and the photosensitivity thereof is insufficient for use in practice.
In line with the recent trend of function-composition and improvement of copying machine, photoconductors capable of coping with a copying machine provided with both analog and digital functions have been developed. More specifically, to cope with the digital function of copying machine, it is desired that the photoconductor be highly photosensitive in a region with a wavelength of light from LD laser power source, namely, in the near infrared region; and to cope with the analog function of copying machine, it is required that the photoconductor be highly photosensitive in a region with the wavelength of white light, namely, in the visible region.
Function-separated laminated photoconductors which have a broad spectral sensitivity ranging from the visible region through the near infrared region are disclosed in Japanese Laid-Open Patent Applications 63-236047, 63-243950, 63-243951 and 1-315752, each of which comprises a mixture of a pigment having photosensitivity in the visible region and a pigment having photosensitivity in the near infrared region.
However, the charge transport layer is generally provided on the charge generation layer comprising the above-mentioned pigments and the charge transporting material contained in the charge transport layer absorbs light, so that the photosensitivity to light with a short wavelength of the function-separated laminated photoconductor is low. Thus, the spectral sensitivities of the conventional laminated photoconductors from the visible region to the near infrared region are not always sufficient for use in practice.
In addition, the laminated photoconductor has the optical problem of generation of interference light because of the presence of an interface between the charge generation layer and the charge transport layer.